barber: heredity in certain micro-organisms. 33 



Thus of 95 filaments, approximately 18 per cent, reverted to 

 the parent type, 11 per cent, partially developed, and 68 per 

 cent, failed to grow at all. Two, or about 2 per cent., grew 

 into permanent new races. Including the 11 per cent., the 

 most of which developed only a few threads, with the 68 per 

 cent., we have 79 per cent, which either failed to grow or de- 

 veloped only slightly. Including all filaments isolated in this 

 series, we have of the total number of over 190 filaments only 

 2, or about one per cent., which formed permanent new races. 

 The number of reversions to type recorded may be too large, 

 because in some experiments selection was made from very 

 young cultures, when there is a greater tendency for normal 

 rods to adhere in filaments than in cultures a few hours older ; 

 and it may be that occasionally a filament was selected which 

 was not a variation, but owed its length to its early age 

 merely. In a large proportion of cases, isolations were made 

 of single normal cells under the same conditions to serve as 

 checks. Of these fully eighty percent, developed. In fact, 

 when these cells were isolated from recent vigorous cultures, 

 as was the case with the larger number of variation isola- 

 tions, they rarely failed to grow. So the failure of the vari- 

 tions to grow cannot be laid to the conditions of cultivation 

 or to injury suffered in the process of isolation. It was 

 fourtd that if non-motile filaments were isolated they almost 

 invariably failed to develop further ; so, during the latter 

 part of the series of experiments, I isolated for the most part 

 motile filaments alone. 



In order to eliminate possible inhibitory effects of concen- 

 tration of medium in hanging drops an additional series of 

 isolations was made in which long filaments were drawn into 

 capillary tubes immediately after isolation. There were 

 about ten in this series and the results obtained were essen- 

 tially like those observed in hanging drops. 



Among the filaments which reverted to the normal type 

 some produced a progeny which for a time showed an abnor- 

 mally large number of long filaments, and some of those 

 which only partially developed produced in a few cases four 

 to six long filaments before growth ceased. There seems to 



3-Univ. Sci. Bull., Vol. IV, No. 1. 



